The Gunning Fog Index (or FOG) is a readability formula developed by Robert Gunning in 1952. It estimates the years of formal education needed to understand a text on first reading. A score of 12 indicates high school senior level; 7–8 is considered ideal for general audiences.

The formula is: Score = 0.4 × [(Words ÷ Sentences) + 100 × (Complex Words ÷ Words)]. We count total words, sentences, and complex words (words with 3 or more syllables), then apply this formula. The result is a grade level from below 6th grade through college.

A score of 7–8 is considered ideal for general audiences. Scores of 6–8 correspond to 6th–8th grade reading level. Scores above 12 are generally too complex for most readers. For business and consumer content, aim for 8 or lower.

Gunning Fog uses sentence length and complex words (3+ syllables), similar to SMOG. Flesch-Kincaid uses all syllables. Gunning Fog tends to produce slightly higher estimates than Flesch-Kincaid. SMOG is preferred for healthcare; Gunning Fog is widely used in business and publishing.

Use Gunning Fog when you need a grade-level estimate for business writing, marketing copy, or general audience content. It works well for passages of 100+ words. If you write for healthcare, SMOG may be preferred. For quick readability checks, Flesch-Kincaid is also popular.